Piloting any aircraft entails knowing its relative speed in relation to the air, that is to say to the relative wind. This speed is determined using static pressure Ps and total pressure Pt measurement probes. The total Pt and static Ps pressures give the modulus of this speed vector. As is known, the total pressure Pt can be measured using a so-called Pitot tube. This is a tube that is open at one of its ends and blocked at the other. The open end of the tube faces substantially into the flow. The air stream situated upstream of the tube is progressively slowed down until it reaches an almost zero speed at the tube inlet. The slowing down of the speed of this air stream increases its pressure. This increased pressure forms the total pressure Pt of the air flow.
The principle of a total pressure measurement probe is recalled by FIG. 1. The probe 10 is intended to be fixed through an opening 11 produced in the skin 12 of an aircraft. The probe 10 comprises a part 13 external to the skin 12 and formed by a Pitot tube 14 borne by a strut 15. The probe 10 also comprises an internal part 16 essentially comprising an electrical connector 17 and a pneumatic connector 18. The connector 17 makes it possible to electrically connect the probe 10 to the aircraft, for example to connect heating means for the de-icing of the probe 10. The connector 18 makes it possible to pneumatically connect the Pitot tube 14 to a pressure sensor or other measurement device, situated inside the skin 12 of the aircraft. The probe 10 is positioned on the skin 12 of the aircraft in such a way that the Pitot tube 14 is oriented substantially along a longitudinal axis of the aircraft, outside the boundary limit, so that the direction of the flow, represented by an arrow 19 substantially faces an inlet orifice 20 situated at a first end 21 of the Pitot tube 14. In the example represented, the Pitot tube 14 is fixed relative to the skin 12 of the aircraft. It is of course possible to mount the Pitot tube 14 on a mobile strut such as, for example, a vane that can be oriented in the axis of the flow as for example described in the patent published under the number FR 2 665 539.
In practice, the air flow can carry solid or liquid particles, such as, for example, water from the clouds, likely to penetrate into the Pitot tube and to build up in the tube at the blocked end. To prevent such a build-up from disturbing the pressure measurement, there are generally provided one or more drain holes and water traps, to avoid any risk of obstruction of the pipe work responsible for transmitting the total pressure to the pressure sensors situated inside the skin of the aircraft or to the instruments of the aircraft instrument panel. As represented in FIG. 2, the Pitot tube 14 thus comprises, in proximity to an end 22, a drain hole 23 making it possible to evacuate particles likely to penetrate inside the tube 14. Still at the end 22 of the tube, a pneumatic line 24 opens into the tube 14 to form therein a pressure tap 40 where the air pressure is to be measured. The pressure tap 40 is generally constructed so as to avoid the ingress of water into the tube 14 and thus form a water trap. The line 24 is, for example, linked to a pressure sensor which is not represented in FIG. 2. The pressure sensor makes it possible to effectively measure the pressure of the air prevailing inside the tube 14 at its end 22. Apart from the drain hole or holes 23, whose sections are small compared to that of the tube 14, the tube is closed at its end 22. The pressure measured at this end therefore represents the total pressure Pt of the air flow.
The drain holes make it possible to evacuate the liquids and any particles that might penetrate into the tube. The slowing down of the air in the tube is not therefore complete and the total pressure Pt measurement is corrupted. More specifically, the greater the efforts made to avoid the build-up of water or of particles of significant size, the more the total pressure measurement is affected by increasing the dimensions or the number of drain holes. Conversely, the greater the efforts to improve the total pressure Pt measurement by reducing the dimensions or the number of drain holes, the more the risk of build-up of water or of particles increases. With the Pitot tube, a trade-off therefore has to be found between quality of the total pressure Pt measurement and risk of disturbing the measurement because of the penetration of water, and of particles conveyed by the air flow where the measurement is performed.
In the operational life of the aircraft, the drain holes can be polluted, because of the ingress of dust, of insects, of plant residues or other foreign bodies. Because of their size and the position of the Pitot tubes on the fuselage of an aircraft, periodically monitoring the integrity of the drain holes is not easy.
The drain holes of the Pitot tubes are generally checked visually. The operator responsible for the maintenance of the airplanes inspects the drain hole or holes using a small lamp. If foreign bodies are observed, the probe is dismantled, and its pneumatic circuits cleaned. This operation becomes all the more difficult when the airplane is of large size. Access to the probe and to the drain holes whose diameter is generally less than 1 mm is difficult.
Also known from the applicant is a monitoring device intended to be temporarily connected to the pressure measurement probe, and that makes it possible to monitor, using an acoustic transmitter and receiver, that the internal cavities and the drain holes of the probe are not blocked. The principle of such a device is notably described by the patent published under the reference FR 2 959 822. It is also reviewed in FIG. 2 of the present application. The monitoring device 25 comprises a transmitter 26 and a receiver 27 intended to be connected to an internal volume 30 of the probe, formed by the interior of the tube 14, the drain hole or holes 23 and the line 24. The transmitter transmits an acoustic signal that is propagated in the internal volume 30 and the receiver is configured to pick up an acoustic signal observed in the internal volume 30. The device also comprises processing means 28 that make it possible to compare the acoustic signal observed in the internal volume to a reference acoustic signal measured on a probe that is not clogged up, in order to establish the presence of particles in the internal volume.
As represented in FIG. 2, the processing means 28 are incorporated in the probe monitoring device. When the difference between the observed signal and the reference signal exceeds a predetermined threshold, the processing means alert the user, for example by means of a lamp 29 mounted on the device. This approach does, however, have limitations. In effect, to improve the reliability of the diagnosis of clogging of the probe, processing algorithms are considered that are more complex than simply comparing a signal difference to a threshold. These more complex algorithms can notably require various interactions with an operator. The incorporation of the processing means in the device connected to the probe mounted on the fuselage of the aircraft make these interactions between the operator and the device difficult. It is therefore desirable to have an apparatus that makes it possible to monitor a pressure measurement probe that is simultaneously reliable, safe and simple for a maintenance operator present in proximity to the aircraft equipped with the probe to be monitored.